Presently, there is a need to mix dry, fine powders with liquids and pasty, sticky sludges. This need exists in the stabilization of FGD sludges with finely ground lime, water and fly ash. Current practice is to utilize pug mill mixers to accomplish this mixing.
A conventional pug mill mixer is essentially a container, or tank, within which two counter rotating pug mill shafts are disposed. A series of pug mill paddles, or "pugs," are attached to the pug mill shafts and radially protrude from the pug mill shafts. Separate streams of constituent ingredients are introduced into the pug mill mixer top at one end and the angled pug mill paddles fold the material as it is mixed and transport the material along the axis of the pug mill shafts to the exit end of the mixer. There, the material is discharged through the bottom of the mixer.
In the past, stabilized FGD sludge has been disposed of, such as by placing the cementitiously hardened product in a landfill. It is known that pug mill mixers in some cases do not produce a homogeneous mixture. For example, material mixed by a conventional pug mill mixer which is subsequently cured typically includes white balls 1/2 inch in diameter, and, in some cases, as large as one or two inches in diameter. These white balls are known to be FGD filter cake which is a highly cohesive, pasty material. The presence of such balls of FGD filter cake in the final cured mass decreases its strength and increases its permeability. Although this problem is often tolerable when the cementitiously hardened product is simply discarded, the lack of homogeneous mixing is intolerable when the FGD sludge is used to make synthetic aggregate or if the final cured landfill material is required to be extremely strong or to have a low permeability.
Pin mixers also have been used to mix dry powders with liquids. The flow of material is along the axis of the pin mixer as with pug mill mixers. Pin mixers are not designed for sticky, cohesive sludges, such as FGD sludge.
In addition, batch mixers have been utilized to mix dry, fine powders with liquids and pasty, sticky sludges. Some batch mixers use small chopper blades or high shear mixer blades in the side of the mixer; however, such mixers are primarily batch type and are relatively expensive.
There still exists a need for a mixer capable of homogenizing dry, fine powders with liquids and pasty, sticky sludges. Preferably, such a mixer should provide a zone of high shear mixing so that the formation of cohesive balls of one of the feed components is minimized. In this way, a mixer would be useful for homogeneously mixing FGD sludge, fly ash, lime and water in a process for making synthetic aggregate. Such a mixer should also be capable of continuous operation.